Radio frequency identification (“RFID”) systems have become very popular in a great number of applications. A typical RFID system 100 is shown in FIG. 1. The RFID system 100 includes an application system 110, a reader 120, and a tag 130. When the tag 130 appears in the operational range of the reader 120, it starts receiving both energy 140 and data 150 via its antenna 133 from the reader 120 via its transmitter/receiver 121 and antenna 123. A rectify circuit 131 in the tag 130 collects and stores the energy 140 for powering the other circuits (e.g., control/modulator 132) in the tag 130. After collecting enough energy 140, the tag 130 may operate and send back pre-stored data to the reader 120. The reader 120 then passes the received response data via a communications interface 160 to the server system/database 111 of the application system 110 for system applications.
The tags 130 in RFID system 100 may be classified into passive and active types according to the power provisions of the tags. Passive tags do not have their own power supply and therefore draw all power required from the reader 120 by electromagnetic energy received via the tag's antenna 133. In contrast, active tags incorporate a battery which supplies all or part of the power required for their operation.
A typical transmission method of energy 140 and data 150 between a reader 120 and a tag 130 in a RFID system 100 is by way of backscatter coupling (or backscattering). The antenna 123 of the reader 120 couples energy 140 to the tag 130. By modulating the reflection coefficient of the tag's antenna 133, data 150 may be transmitted between the tag 130 and the reader 120. Backscattering, as shown in FIG. 2, is typically used in microwave band RFID systems. Power Pin 210 is emitted from the reader's antenna 123. A small proportion of Pin 210 is received by the tag's antenna 133 and is rectified to charge the storing capacitor in the tag 130 for serving as a power supply. After gathering enough energy, the tag 130 begins operating. A portion of the incoming power Pin 210 is reflected by the tag's antenna 133 and returned as power Preturn 220. The reflection characteristics may be influenced by altering the load connected to the antenna 133. In order to transmit data 150 from the tag 130 to the reader 120, for example, a transistor may be switched on and off in time with the transmitted data stream. The magnitude of the reflected power Preturn 220 may thus be modulated and picked up by the reader's antenna 123.
In backscattering RFID systems, a number of tags 130 interact with a main reader device 120 as shown in FIG. 3. The reader 120 is used to: (i) power up the tags 130 via the power of the RF signal; (ii) transfer data to the tags 130; and, (iii) read information from the tags 130. Amplitude shift keying (“ASK”) modulation is typically used in RFID systems 100. In ASK modulation, the amplitude of the carrier is switched between two states controlled by the binary transmitting code sequence. Also, in some applications, phase shift keying (“PSK”) modulation may also be used.
One problem with existing RFID systems is that they have limited capability with respect to ranging or determining the range or location of a tag or object.
A need therefore exists for an improved method and system for identifying and finding a range of an object. Accordingly, a solution that addresses, at least in part, the above and other shortcomings is desired.